U.S. Pat. No. 4,285,731 discusses the hazards of lead and cadmium release from glazes and enamels containing those components and discloses frit compositions which are substantially free from those materials. Those frits consisted essentially, in weight percent on the oxide basis, of:
SiO.sub.2 : 35-47 PA1 B.sub.2 O.sub.3 : 5.5-9 PA1 BaO: 24-42 PA1 TiO.sub.2 : 1.5-4 PA1 ZrO.sub.2 : 6-10 PA1 Li.sub.2 O: 1-5 PA1 SrO: 0-8 PA1 MgO: 0-5 PA1 CaO: 0-4 PA1 ZnO: 0-10 PA1 Bi.sub.2 O.sub.3 : 0-8 PA1 SrO+MgO+CaO+ZnO+Bi.sub.2 O.sub.3 : 0-10. PA1 SiO.sub.2 : 51-60 PA1 B.sub.2 O.sub.3 : 4.5-8 PA1 BaO: 0-13 PA1 SrO: 0-18 PA1 BaO+SrO: 6-30 PA1 ZrO.sub.2 : 4-8 PA1 Al.sub.2 O.sub.3 : 5-8 PA1 Li.sub.2 O: 0-4 PA1 Na.sub.2 O: 0-5 PA1 K.sub.2 O: 0-5 PA1 Li.sub.2 O+Na.sub.2 O+K.sub.2 O: 1-5 PA1 MgO: 0-6 PA1 CaO: 0-12 PA1 Bi.sub.2 O.sub.3 : 0-10 PA1 MgO+CaO+Bi.sub.2 O.sub.3 : 0-20. PA1 Li.sub.2 O: 3-4 PA1 Na.sub.2 O: 0.75-3 PA1 BaO: 3.5-9.5 PA1 B.sub.2 O.sub.3 : 14-17.5 PA1 Al.sub.2 O.sub.3 : 6.75-8.75 PA1 SiO.sub.2 : 48-55 PA1 ZrO.sub.2 : 6.75-10.5 PA1 F: 3-4. PA1 SiO.sub.2 : 50-54 PA1 Al.sub.2 O.sub.3 : 5-8 PA1 B.sub.2 O.sub.3 : 6-12 PA1 CaO: 4-6 PA1 Li.sub.2 O+Na.sub.2 O+K.sub.2 O: 4-6 PA1 MgO: 2-8 PA1 BaO: 2-5 PA1 SrO: 5-8 PA1 ZnO: 1-2. PA1 SiO.sub.2 : 48-54 PA1 Al.sub.2 O.sub.3 : 7-11 PA1 B.sub.2 O.sub.3 : 16.5-20 PA1 BaO: 11-14 PA1 CaO: 2-3 PA1 ZnO: 2-2.5 PA1 Na.sub.2 O: 4.25-5.25 PA1 K.sub.2 O: 0.4-1. PA1 SiO.sub.2 : 29-55 PA1 B.sub.2 O.sub.3 : 7-31 PA1 Al.sub.2 O.sub.3 : 2-8 PA1 ZrO.sub.2 : 5-16 PA1 Na.sub.2 O: 4-20 PA1 Li.sub.2 O: 0-7 PA1 Na.sub.2 O+Li.sub.2 O: 6-24 PA1 F: 0.75-4.
As is employed generally in the ceramic art and as used herein, glazes are deemed to be transparent glasses which may be colored, and enamels as glazes containing particles of pigments to provide color thereto. Both types of products are applied to the surface of a glass, glass-ceramic, or sintered ceramic body which is to be decorated in the form of very finely-divided particles, conventionally termed "frit", and that powder then fired to fuse to and form a strongly-bonded, continuous film on the surface of the body.
The above frits exhibited a coefficient of thermal expansion (20.degree.-300.degree. C.) between about 65-75.times.10.sup.-7 /.degree.C., a viscosity suitable for firing at about 700.degree.-950.degree. C., and excellent resistance to attack by acids and bases. That patent also recited four criteria which must necessarily be demonstrated by glazes and enamels, and referred to an optional characteristic demanded when a transparent, highly glossy decorative coating is desired.
First, the firing or maturing temperature of the frit, i.e., the temperature at which the frit will flow sufficiently to form a smooth uniform coating, must be low enough to forestall thermal deformation of the substrate being coated.
Second, the coefficient of thermal expansion of the frit must be compatible with that of the ware being coated to preclude crazing and/or spalling; the most advantageous frits having coefficients of thermal expansion somewhat lower than that of the substrate such that, when the ware is cooled to room temperature, the fired coating will form a surface compression layer.
Third, the frit must manifest excellent resistance to attack by acids and bases since corrosion of the coating can result in the loss of gloss, the development of haze and/or iridescence, the formation of porosity, or other defects deleterious to the appearance or physical character of the coating.
Fourth, the frit must maintain good glass stability; i.e., the frit must not devitrify to any substantial extent during the maturing firing.
Fifth, where a glossy appearance is desired, the refractive index of the frit must be high.
Where the glazes or enamels are to be utilized in food service applications, such as culinary ware and tableware, they must resist attack by acids present in foods and alkalies present in detergents employed in dishwashers. The extreme toxicity of certain heavy metals, notably lead and cadmium, has been recognized by the Food and Drug Administration (FDA). Hence, in its Compliance Guidance Manual, issued June 13, 1974, that agency established maxima for cadmium and lead release which food-containing surfaces must not exceed.
However, as was pointed out in U.S. Pat. No. 4,285,731, PbO and CdO serve two very useful functions in glazes; viz., to soften the frit (lower its melting point), and to raise the refractive index of the frit. Accordingly, the removal of PbO and/or CdO from a frit to eliminate the potential for CdO and/or PbO toxicity must be undertaken with due regard to satisfying the above criteria for enamels and glazes.
U.S. Pat. No. 4,282,035 also describes lead and cadmium-free frits, those frits exhibiting coefficients of thermal expansion (20.degree.-300.degree. C.) between about 52-65.times.10.sup.-7 /.degree.C., maturing temperatures of about 850.degree.-1100.degree. C., excellent resistance to attack by acids and alkalies, and consisting essentially, in weight and the oxide basis, of:
U.S. Pat. No. 4,311,504 discloses applying a mixture of a frit having a composition within the ranges set forth in U.S. Pat. No. 4,285,731 and cadmium sulfoselenide onto the surface of a glass, glass-ceramic, or sintered ceramic body and firing the body at about 700.degree.-950.degree. C. in a non-oxidizing or inert environment to produce a bright red coating.